While alternating current (AC) has long been the standard for distributing power, particularly to loads in fixed locations that may be widely separated from power generation sources where power is continuously generated, there is increasing interest in power systems using so-called renewable resources such as solar and wind power where power generation may be intermittent and generated power, when available, must be stored, often as the potential energy of stored charge, until needed. The same is true for portable or mobile devices which must generally contain their own power sources, such as electrical generators and/or batteries which store energy as charge and deliver that energy as direct current (DC). If alternating current is required by the ultimate load, so-called power inverters can be used to develop AC power from a DC power input. On the other hand, batteries are often not an economical source of portable power unless they are capable of being recharged using energy which is ultimately generated and distributed as alternating current.
Among applications where the powered device is mobile and energy must necessarily be intermittently stored, there has been much recent interest in using electrical power for powering vehicles in order to avoid or at least reduce atmospheric pollution, particularly in areas where numerous vehicles are concurrently in use. Therefore, there has been a corresponding interest in various technologies for batteries which can store greater quantities of energy in order to increase the range of the vehicles as well as the capability of such batteries to be quickly recharged; ideally, within a period time comparable to that required for filling the fuel tank of a vehicle powered by an internal combustion engine. Thus, variable voltage may be necessary for both rapidly charging batteries and to prolong battery life as well as to accommodate various safety concerns encountered in some technologies. Further, isolation of the battery from the power source is generally desirable, particularly for reasons of safety.
In summary, particularly for charging batteries capable of storing relatively large amounts of energy, it is generally desirable to provide an isolated DC-DC converter capable of operating in either buck or boost mode in order to provide an output voltage which is variable over a wide range from a relatively constant input voltage and which is capable of bi-directional power flow. Both buck and boost modes are desirable since input voltage may be higher or lower than the desired output voltage. In the case of battery charging, the output voltage desired depends of the state of battery charge; At he beginning of battery charging the voltage could be very low but near full charge, the voltage is high. Bi-directional power flow capability is desirable since, in addition to transferring power from the grid or a local generator to a battery, the battery may be required to provide energy to the grid or other devices or to another battery.
It is also generally desirable for the DC-DC converter to be controllable to produce a variable output voltage in a simple manner that does not require complex circuitry to produce suitable control waveforms and which has so-called soft switching capabilities (e.g. where switching is performed when voltage and/or current is low or zero) in order to achieve high efficiency. It is also very desirable for high efficiency to be maintained over a wide range of load conditions since the load presented by a battery under charging conditions may be very large when the battery has been deeply discharged but may be much smaller as the battery approaches full charge. In this latter regard, it is also very desirable for the output voltage of the converter to be controllable in a predictable manner; requiring that the controlled output voltage to be substantially unaffected by the load which the battery presents.
Unfortunately, power converter circuits developed prior to the present invention have not been able to achieve all of these desirable features. In particular, known isolated DC-DC converters capable of bi-directional power flow have only been capable of providing soft switching over a very narrow load range and have exhibited load-dependent voltage gain even when complicated control schemes and circuitry are employed.